In spite of a stable decline in the incidence of tuberculosis in countries participating to control surveys, there were an estimated 8.8 millions new cases and 1.6 million deaths in 2005. In fact, numbers of individuals succumbing to tuberculosis have vastly increased as a result of the HIV/AIDS pandemic, and increased mobility owing to global travel has increased the transfer of virulent and multidrug-resistant tuberculosis. Compared with other infections there are relatively few antimicrobial agents that are clinically active against Mycobacterium tuberculosis. Prolonged antibiotic treatment is also required, as the bacterium can enter a dormant, antibiotic-resistant phase.
Except for very young children, few people that have been infected with Mycobacterium tuberculosis become sick immediately after tuberculosis bacteria enter in their body (primary infection). Many tuberculosis bacteria that enter the lungs are immediately killed by the body's defences. The bacteria cells that survive are captured inside macrophages. The captured bacteria can remain alive inside these cells in a dormant state for many years, walled off inside tiny scars (latent infection). In 90 to 95% of cases, the bacteria never cause any further symptom. Nevertheless, in about 5 to 10% of infected people bacteria cells start to multiply and these people actually enter into the symptomatic disease state. It is in this active phase of the tuberculosis disease that an infected person actually becomes sick and can spread the disease.
Infection with Mycobacterium tuberculosis usually results in so-called primary tuberculosis or Ghon's complex in approximately 90% of the individuals. This usually limited infection comprises a focal multiplication of the Mycobacterium in the lung tissue in association with lymphangitis, infection, and massive enlargement of the corresponding hilar draining lymph node. In approximately 95% of these individuals, the inflammatory reaction is spontaneously contained and in many cases calcifies and persists for the remainder of the person's life.
In more than half of the infected individuals, activation of dormant bacteria happens within the first 2 years following the time of infection, but this activation may not occur for a very long time, in some cases for up to 5 years following the time of infection. Activation of dormant M. tuberculosis bacterium cells often occurs when the individual's immune system becomes impaired—for example, from very advanced age, the use of corticosteroids, or AIDS. Like many infectious diseases, tuberculosis spreads more quickly and is much more dangerous in people who have a weakened immune system. For such people (including the very young, the very old, and those who are also infected with HIV), tuberculosis can be life threatening.
Thus, notwithstanding the clinical resolution of Ghon's complex, several M. tuberculosis organisms remain viable for life, a condition known as latent or dormant tuberculosis. Primary tuberculosis usually stimulates strong and long-lasting cellular immune response to M. tuberculosis antigens, which can be detected even years later by the delayed-type hypersensitivity skin test (the purified protein derivative [PPD] or Mantoux test). Reactivation of latent tuberculosis leading to chronic pulmonary tuberculosis or adult tuberculosis may occur afterwards, even decades later, due to intervening events such as malnutrition or immunosuppression. However, clinical and molecular evidence indicates that adult tuberculosis occurs most often due to exogenous reinfection with M. tuberculosis in geographical areas where the rate of contagion is high, such as in many developing countries. Although the general consensus is that adult tuberculosis caused by reactivation of latent infection is associated with some predisposing immunodeficiency, the need for such an intervening condition for disease development due to exogenous reinfection of PPD-positive healthy immunocompetent individuals is still a controversial issue. Unfortunately, experiments designed to study this controversy have been hampered by the lack of a reliable animal model of dormant infection. Moreover, such an animal model is in high demand for validation of vaccine candidates in the context of pre-sensitization with M. tuberculosis antigens, a condition highly prevalent among the human population in need of an antituberculosis vaccine.
The treatment of M. tuberculosis infections requires at least six months of antimycobacterial therapy with the association of multiple drugs. This long duration of treatment is justified by the poor efficacy of available antibiotics, including the main drugs isoniazid and rifampin, against dormant M. tuberculosis bacilli that are thought to persist in particular environment such as granuloma or caseum. In vitro models that mimic the persistent state have been developed based on nutrient starvation, oxygen deprivation and exposure to nitric oxide. These models showed that non-replicative and low metabolic states of the bacteria could be responsible for the poor in vivo response to currently available drugs.
There is thus a need in the art for novel antibacterial substances for treating infection of individuals with Mycobacterium tuberculosis. Importantly, there is a need in the art for novel compounds that are active against the dormant state of M. tuberculosis, so as to prevent activation of the bacteria cells in the infected individuals, especially in immuno-compromised individuals for whom entering into the clinical phase of tuberculosis would be life-threatening.
Several attempts have already performed in the art for identifying antibacterial substances having bactericidal properties against Mycobacterium tuberculosis cells in the dormant state.
Notably, Hu et al. have disclosed the in vitro bactericidal activity of pyrazinamide, a sterilizing substance, against M tuberculosis cells cultivated in starvation conditions mimicking cells in dormant state (Hu et al., 2006, International Journal of tuberculosis and lung disease, Vol. 10(no3): 317-322). These authors have shown that pyrazinamide possess an increased bactericidal activity towards M tuberculosis bacteria with low metabolic activity, as compared with normal bacterial cells.
Also, Murphy et al. have taken benefit from genome-wide M tuberculosis gene expression data for identifying several genes that were found up- or down-regulated specifically in bacterial cells under simulated dormancy conditions (Murphy et al., 2007, BMC Infectious diseases, Vol. 7: 84). These authors have determined that promising targets for drug discovery included several regulatory elements (devR/devS, relA, mprAB) enzymes involved in redox balance and respiration, sulfur transport and fixation, pantothenate, isoprene and NAD biosynthesis.
However, there is still a need for novel M tuberculosis bactericidal substances, especially substances which are active against the dormant state of M tuberculosis bacteria cells, as well as to methods for the screening such bactericidal substances.